The present invention relates generally to aircraft safety devices and, more particularly, to a method and apparatus for automatically limiting adverse yaw and roll caused by asymmetrical thrust due to engine failure in a multiengine aircraft.
In the typical fixed-wing multiengine airplane configuration, where equal net thrust is being generated by engines located along each wing structure, any tendency for "yaw" about the vertical axis of the aircraft is cancelled. However, where one engine produces significantly more thrust than the corresponding engine on the opposite side of the aircraft, several effects take place.
First, the aircraft will tend to yaw in the direction of the engine producing lower thrust because of the greater torque generated about the vertical axis by the engine producing the greater thrust. This effect is compounded in an "engine failure" situation where an inoperative engine produces additional drag while windmilling and until it is feathered.
A second effect of this asymmetrical thrust configuration is to cause the aircraft to roll in the direction of the lower thrust producing or inoperative engine, due in part to the decrease in lift produced by the wing on which it is located. The turning tendency of the aircraft about its vertical axis toward the inoperative engine due to asymmetrical thrust created by the operating engine may be overcome by a counteracting moment produced with the rudder. When the rudder is fully deflected, the corrective turning moment created by the rudder about the aircraft vertical axis is dependent upon the velocity of air flow across the rudder which, in turn, is dependent on the air speed. As the aircraft decelerates, a speed is reached below which the rudder turning moment will no longer be sufficient to balance the thrust moment, and directional control will be lost.
The term V.sub.mc is defined as the minimum airspeed at which an airplane is controllable when a critical engine is suddenly made inoperative and the remaining engine is producing takeoff power. The critical engine is defined as the engine which, if it suddenly fails, most adversely affects aircraft performance. V.sub.mc thus represents the minimum control speed of an aircraft with a critical engine inoperative.
In a situation where one engine fails with full power applied to an operative engine comparably disposed on the opposite side, as the airspeed drops below V.sub.mc, directional control is lost and the airplane tends to roll as well as yaw into the inoperative engine. This rolling tendency is aggravated as airspeed is further reduced since the roll must be counteracted by use of aileron control, producing aileron-induced yaw. If a stall should occur in this condition, a violent roll into the inoperative engine may be experienced. Because of the possible disastrous consequences of inducing such a violent roll, airspeed must be maintained above V.sub.mc at all times during single-engine operation of a multiengine aircraft. However, if the air speed should fall below V.sub.mc for any reason, power must be reduced on the operative engine to regain control of the aircraft. This procedure for regaining control of the aircraft is emphasized in the Flight Training Handbook, AC 61-21A (1980) published by the Federal Aviation Administration, Flight Standard Service.
While devices are available to partially compensate for an engine failure by automatically controlling aircraft aerodynamic surfaces, such devices lose effectiveness and can even exacerbate the problem as airspeed falls toward V.sub.mc. For example, U.S. Pat. No. 4,143,839, to Antonov el al, describes an apparatus which automatically deploys spoilers mounted outboard on the wings to counteract the rolling tendency of an aircraft when one of its propulsion units fails. U.S. Pat. No. 3,160,367, to J.A.M. Lecarme, discloses a system for turbine-powered aircraft, which uses a bleed air system to effect automatic trimming of the rudder under asymmetric power conditions. U.S. Pat. No. 2,954,944, to F. Huet, describes a device to increase aileron effectiveness to control aircraft rolling tendency in an engine failure situation by automatically blowing air along the upper surface of a control surface. Because these devices utilize aircraft control surfaces to compensate for asymmetrical thrust, their effectiveness diminishes with reduced air flow as the airspeed drops toward V.sub.mc.
A need, therefore, exists for safety apparatus that does not depend upon manipulation of aircraft control surfaces but, instead, directly abates the problem caused by engine failure at low airspeed.